This invention relates to a method of manufacturing new and improved noise attenuating panels and more particularly to a sandwich panel having a central cellular core having endwise directed cells with an imperforate sheet bonded thereto on one surface and the perforation exposed surface of a perforate facing sheet with a layer of porous material bonded thereto bonded to the opposite core surface, wherein there is continuous communication between the core cells and the atmosphere adjacent to the noise to be attenuated. The product manufactured under the method of the invention is specifically suitable for use in a severe environment, such as, for example, lining the high velocity surfaces of an aircraft turbo fan engine.
In manufacturing sound attenuating metal honeycomb sandwich panels which are exposed to an extreme environment and are utilized to suppress a wide range of noise sound frequencies, it is common practice to provide a cellular structure utilizing the Helmholtz resonance principle, wherein a first imperforate sheet of material is bonded to one core surface of a sheet of cellular core material and a thin perforate sheet of like material is applied to the opposite core surface.
Panels having this general type of construction, although satisfactory for attenuating some specific sound frequencies, are found to be inefficient noise attenuators for a broad range of frequencies customarily encountered in and around modern aircraft jet engines. Additionally, it has been found that the perforations of the perforated sheet when exposed directly to a high speed gas flow across their surface create turbulence to that flow. Other known concepts have included interposing a sheet of fibrous material between the perforate sheet and the core surface. This has proven to be unsound structurally when used in severe environments for providing both attenuation and structural integrity or on exterior aircraft surfaces exposed to high speed inflight air flow.
Attempts to successfully manufacture various other sound attenuation materials of this general type have been unsuccessful in that the adhesive used for bonding the perforate sheet to the core will without exception ooze or wick into the perforations of the perforated sheet during assembly and at least fill some of the perforations. The blocking off of the perforations reduces the effective open area of the perforated sheet which in turn increases the desired flow resistance between the source of the sound to be attenuated and the cells of the core. When the number of perforations or the perforation size is increased to overcome the blocking of some of the perforations to maintain the desired flow resistance, the structural strength of the sandwiched structure is correspondingly reduced and the flow turbulence thereacross is increased. In those structures which include porous fibrous material positioned between the perforate sheet and the core structure, the adhesive wicks into the pores of the fibrous material in addition to the blocking of some perforations, thus further increasing the flow resistance and reducing the attenuation effectiveness of the structure.